Emit a warning if no sources are passed to ProtoData CLI

[alexander-yevsyukov]

During the migration to new version of Protobuf library it was discovered that the target directory of protoc changed.

It used to be /build/generated/source/proto/<plugin-name>.

In v4.x it became /build/generated/sources/proto/<plugin-name>, i.e., plural for sources.

Since the path to the Protobuf-generated code was a constant in our code-base ProtoData CLI did not discover any source code in the expected location. And it exited silently with normal exit code.

It took remote debugging session to discover the root cause of absence of generated files.

Obviously, if we have ProtoData applied to the project, we expect Protobuf code generated. And if there's no such code, the ProtoData CLI should emit a message into console output to warn the programmer about unusual situation.